Rowling used both sides of her card to hand-write a prequel to her seven-book Harry Potter saga, while Lessing penned a story about the power of reading. Stoppard wrote a short mystery and Atwood was due to fill out her card remotely using a robotic arm controlled by computer linkup.

The cards will go on sale at the "What's Your Story?" auction at Waterstone's flagship store in central London on June 10. The proceeds are to go to English PEN, the writers' association, and the British charity Dyslexia Action. Copies of the cards will be collated into a book to be made available at the bookstore and online in August.

Organizers refused to comment on the content of Rowling's Harry Potter prequel, but Hogwarts fans hoping for another book to add to their collection may be disappointed by her signoff.

"From the prequel I am not working on -- but that was fun!" Rowling wrote.

Rowling has previously said she had no plans to write another Potter novel, but in December she sold a handwritten, leather-bound book of fairy tales she described as drawing on